408 Charles R. Stockard. 
the ectoderm also lack a lens, as is illustrated on the left of figs. 
27 and 28. On the other hand, it is remarkable how small and 
ill-formed an optic cup-like structure has the power of stimulating 
a lens to arise from the ectoderm. Figs. 2, 23 and 24 and plate 
II, fig. 5, show small choroid cups with no retinal differentiation 
whatever, yet closely associated with perfectly formed lenses. In 
fig. 21 is seen an extremely defective cup with a small lens; a 
larger independent lens is shown on the eyeless side. Fig. 22 
illustrates an extremely insignificant eye-like body buried within 
the brain, yet close by is a small crystalline lens; these are the 
only eye parts found in thisembryo. In fig. 19, a section through 
the eye of an incomplete cyclops, each component of the eye has 
a lens, while in fig. 20 the eye components are closer together and 
in fig. 15 further apart, yet each of these possesses only a single 
lens, although it is elongate in fig. 20. It is difficult to say why 
such eyes occasionally possess two lenses. After an examination 
of a large number of such eyes no general rule is found. It may 
be due in some way to the manner in which the optic cup periph- 
ery meets the ectoderm, whether as a circle, an oval or at times 
a much constricted oval so that two areas of ectoderm are sepa- 
rately stimulated to form lenses. 
This consideration forces the conclusion that an optic cup at 
some stage in its development, whether normal or defective, 
invariably possesses the power to stimulate lens-formation from 
the ectoderm with which it comes in contact. 
e May the Optic Vesicle Cause Lens-Formation from Ectoderm 
Other than that which. Normally Forms a Lens? 
This question is answered by the cyclopean monsters. It is 
scarcely conceivable that the ectoderm which would normally lie 
over the lateral eyes has the power to migrate, or follow the optic 
vesicle so exactly as always to lie just over the vesicle wherever 
it may chance to develop. Many embryos have eyes in unusually 
anterior positions and derive their lenses from anterior ectoderm, 
while others possess ventral eyes with lenses derived from ventral 
ectoderm. It occurs in a few cases, as the writer previously 
